Force looking for the [JniAddNativeMethodRegistration] attribute

[grendello]

Context: dotnet/android#4302
Context: #582

Java.Interop tests use their own runtime which doesn't know about the
results of Xamarin.Android's build process scanning for the presence of
the JniAddNativeMethodRegistration custom attribute. In addition,
Java.Interop's Java type scanner (for typemaps) does not include in its
results the types in Java.Interop-Tests which derive from
Java.Interop.JavaObject (which is functionally equivalent to
Xamarin.Android's Java.Lang.Object) and, thus, the scan doesn't notice
that the custom attribute mentioned above is used by a few test classes.
In effect, two tests fail with a similar exception:

Java.Lang.LinkageError : No implementation found for void com.xamarin.interop.CallVirtualFromConstructorDerived.calledFromConstructor(int) (tried Java_com_xamarin_interop_CallVirtualFromConstructorDerived_calledFromConstructor and Java_com_xamarin_interop_CallVirtualFromConstructorDerived_calledFromConstructor__I)
  at Java.Interop.JniEnvironment+InstanceMethods.CallNonvirtualVoidMethod (Java.Interop.JniObjectReference instance, Java.Interop.JniObjectReference type, Java.Interop.JniMethodInfo method, Java.Interop.JniArgumentValue* args) [0x0008e] in <d30c11568ddc48d5aff8f2d4b2eaa867>:0
  at Java.Interop.GenericMarshaler.JniPeerInstanceMethodsExtensions._InvokeConstructor[T] (Java.Interop.JniPeerMembers+JniInstanceMethods peer, System.String constructorSignature, Java.Interop.IJavaPeerable self, T value) [0x00059] in <f6f553d46eb149a29e6542964ef27aaa>:0
  at Java.Interop.GenericMarshaler.JniPeerInstanceMethodsExtensions.FinishGenericCreateInstance[T] (Java.Interop.JniPeerMembers+JniInstanceMethods peer, System.String constructorSignature, Java.Interop.IJavaPeerable self, T value) [0x00029] in <f6f553d46eb149a29e6542964ef27aaa>:0
  at Java.InteropTests.CallVirtualFromConstructorBase..ctor (System.Int32 value) [0x0004e] in <d39267c627f54df3bc31406b36effb6c>:0
  at Java.InteropTests.CallVirtualFromConstructorDerived..ctor (System.Int32 value) [0x00000] in <d39267c627f54df3bc31406b36effb6c>:0
  at Java.InteropTests.InvokeVirtualFromConstructorTests.InvokeVirtualFromConstructor () [0x00000] in <d39267c627f54df3bc31406b36effb6c>:0
  at (wrapper managed-to-native) System.Reflection.RuntimeMethodInfo.InternalInvoke(System.Reflection.RuntimeMethodInfo,object,object[],System.Exception&)
  at System.Reflection.RuntimeMethodInfo.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x0006a] in <26b1d17e27e54492bd3e4105b16bfc7e>:0
  --- End of managed Java.Lang.LinkageError stack trace ---
java.lang.UnsatisfiedLinkError: No implementation found for void com.xamarin.interop.CallVirtualFromConstructorDerived.calledFromConstructor(int) (tried Java_com_xamarin_interop_CallVirtualFromConstructorDerived_calledFromConstructor and Java_com_xamarin_interop_CallVirtualFromConstructorDerived_calledFromConstructor__I)
  at com.xamarin.interop.CallVirtualFromConstructorDerived.calledFromConstructor(Native Method)
  at com.xamarin.interop.CallVirtualFromConstructorBase.<init>(Unknown Source:35)
  at com.xamarin.interop.CallVirtualFromConstructorDerived.<init>(Unknown Source:2)
  at crc64f295cabbf85394f5.TestSuiteInstrumentation.n_onStart(Native Method)
  at crc64f295cabbf85394f5.TestSuiteInstrumentation.onStart(Unknown Source:0)
  at android.app.Instrumentation$InstrumentationThread.run(Instrumentation.java:2189)

Force the tests to always assume the custom attribute is present, thus
restoring the default behavior from before Java.Interop PR582